The equipment used at Astro-Sun for observing and taking pictures of the sun

Powerful instruments, well dimensioned to work together

Notice

Never look directly at the Sun with the naked eye or with any optical device without a filter specifically designed for this purpose, such as binoculars or a telescope. Even a brief flash of sunlight, especially under magnification, can cause serious eye damage.

In the following, we describe some tools we use for solar observing but do not go into detail and should this NOT be used to make your own telescope and filter settings.

The telescope

We used a telescope with an aperture of 152 mm and a focal length of 1200 mm. Observing the sun with such an aperture is really unusual because of the need to get almost perfectly parallel light rays through the H-apha filter. It is a bit technical and we will be happy to explain it to you. In short, the sun is usually observed with very small aperture telescopes, i.e. with small lenses of mirror, because these telescopes work well with the filter that allows observation. However, at Astro-Sun we know that a small aperture is always synonymous with low image quality, so we have opted for a rather large telescope, adapting filters and special lenses to achieve high quality photographs of the sun. In short, we make an effort so that you can enjoy an unforgettable experience.

The sun's rays are very powerful and easily reflect off the walls of the telescope shortly before they reach the lens or the camera. To avoid this, we have coated the inside of the eyepiece holder with an extremely "black" paint. We used Musou paint, known to be one of the blackest paints in the world. This paint reflects very little light.
  • Musou special paint
  • Musou special paint

A usual black paint looks grey in comparison to Musou paint.

The inside of the eyepiece holder was painted with a low quality paint that reflected a lot of light. By painting it with Musou paint, the results are immediate.

 

The AstroSolar film

The AstroSolar film is used for observing the sun (not for taking photographs). It is placed in full aperture, i.e. in front of the telescope. It is the first thing that the sunlight passes through, before it passes through the first lens of the telescope. This film transmits only a fraction of a hundred thousand of the sun's light, and in particular prevents 100% of ultraviolet (UV) and infrared (IR) rays from passing through.

It sounds great, but let's be honest. This filter has a wide bandwidth in the visible spectrum, i.e. it does not filter out a certain wavelength. The result is that it is ideal for observing the sun, but you won't see much detail either. You will only see the perfect sphere of the sun with its typical black spots.

It is an interesting didactic tool, but using a different kind of filter and a camera, we want to reach much more.

 

The Quark Chromosphere filter

This powerful filter manufactured by Daystar is the heart of our high quality sun imaging system.

Visible light from the sun has wavelengths ranging from 3800 Å to 7000 Å, i.e. from blue to red (Angstroms, Å is a very small unit of distance, 100 trillionths of a metre, and wavelength is the distance between two peaks of the light wave). The filter will select a small part of this spectrum around 6562.8 Å.

The bandwidth is only about one Angstrom, which is very narrow indeed. Only sunlight with a wavelength of 6562.8 Angstroms ± 0.5 Å will pass through the filter. This wavelength, called "H-alpha", is very usual in astronomy. In short, the hydrogen atom, because of its electronic configuration, tends to emit or absorb this wavelength. When we observe the sun by selecting the "H-alpha" light, we discard almost all the sunlight and we only keep what is interesting, which will allow us to see details.

Rectification of the incidence of the sun's rays

As a result of the large aperture of the telescope for solar observation, the sun's rays exit the telescope at a certain angle of incidence. However, in order to pass through the H-alpha blocking filter, the light rays must be almost perfectly parallel to each other. For this reason we had to correct the angle of the rays with a telecentric lens.

The telecentric lens causes the light rays to exit parallel to each other. The image formed of the object in a certain plane does not depend on the distance of that plane from the lens.

 

Blocking filter protection

Since we are using a telescope with a large aperture, we will also use two other filters before the Daystar blocking filter to remove the light we don't need (although with a much wider bandwidth). This allows us in particular to remove infrared that could heat up and damage parts of the telescope or the Daystar filter itself.

Caution: these two filters do NOT constitute protection for the human eye. The sunlight passing through these two filters is amply sufficient to damage the eye in a fraction of a second. In order to be able to observe the sun, either the AstroSolar foil or the H-alpha blocking filter must be correctly installed on the telescope.

 

The camera

For photography using a telescope, rather than a camera, it is usually just the chip camera that is used. The telescope, with its lenses, is the lens of our camera (chip + telescope).

The camera that we have to use for sun observation has to be very, very fast. The reason is that we want to get an image of the sun in a very short time because if we wait any longer, the image will be blurred by air movements in the atmosphere. In full sunlight, the air movements in the atmosphere are much more important than at night. These are movements (generally upward and downward) of air masses with different temperatures. The speed of light depends on the medium and in this case it depends on the temperature of the air, so that when light passes from one medium to another, a phenomenon called refraction occurs. Light rays are not straight and the image of the sun is always distorted. As air masses move, if the exposure time is long, the image is blurred instead of distorted, which is much worse.

A blurred image cannot be fixed. But a distorted image, in a way, can. In fact, if instead of taking a single photograph we take thousands of them, all distorted in a different way, the computer will be able to select the patterns that are repeated in the thousands of photographs and that correspond to an undistorted area. In short, what we do is take a fairly poor quality video of the sun, and the computer will extract a nice image from the video.

The camera we use at Astro-Sun is capable of shooting 164 images per second at a resolution of 1936x1216. The images are immediately downloaded via USB3.0 cable to the hard disk of a laptop. No matter how much capacity is the hard drive, it will soon fill up if we make a too long video. In a few seconds, we will create a video of several gigabits.

As you can see, the video is of poor quality. But the computer will be able to extract a high quality photograph from this video.

Note that the camera we use is monochrome. We work in black and white. This is normal because before the camera we used a filter which purpose was to select a very precise wavelength of sunlight. Behind this filter, it makes no sense to talk about colours. The camera takes a black and white video of a sun that looks red and black. What is important here is speed. And yes, the colours of the sun that you see on the front page of this website are "fictitious". No technology would be able to take detailed photographs of the sun in true colour. The details of the sun always exist at a certain wavelength, and are therefore always monochrome.

 

Equatorial mount of our own manufacture

The inclination of the earth's axis with respect to a perpendicular to the ecliptic plane is 23°27'. Throughout the seasons the angle of the sun to the equatorial plane of the earth varies between +23°27' and -23°27'. A telescope for observing the sun only needs a range of movement of +23°27' and -23°27' declination. We have taken advantage of this to build a mount with less declination in exchange for much more rigidity. This mount is equipped with motorisation for tracking.